A trend of utilizing Internet of Things (abbreviated to “IoT”) is appearing in various industries. For example, in the manufacturing industry, an example of providing sensors for sensing manufacturing apparatuses and workers in a plant, transmitting sensing information to a management apparatus (gateway) or the like through a local network (a wired local area network [LAN] or a wireless LAN) or the like, and performing monitoring of a state of the manufacturing apparatuses and flow line analysis of the workers is known. Further, an example of transmitting data collected at a gateway to a cloud through a wide-area network and performing analysis of the collected data on the cloud side is also known.
When various sensors and the like are installed in a plant, not only a wired network such as Ethernet (registered trademark) but also wireless networks using standards such as industrial wireless standards of Wireless Highway Addressable Remote Transducer (HART) and International Society of Automation (ISA) 100.11a, a wireless local area network (LAN) in compliant with Institute of Electrical and Electronic Engineers (IEEE) 802.11 standards, Bluetooth (registered trademark), and ZigBee are used. It is known that both of a wired network and a wireless network have advantages and disadvantages. For example, either of a wired and a wireless network is selected on the grounds of a number of sensors installed in a plant, whether or not a wired network can be physically run to the sensors, an introduction cost, and the like.
As is well known, when an interference source affecting wireless communication exists in a wireless network, communication quality is degraded. For example, in a case of a wireless LAN using a frequency in a 2.4 gigahertz (GHz) band, such as IEEE 802.11b, the frequency band is a frequency band called an Industry Science Medical (ISM) band. Accordingly, the frequency band can be used for various purposes other than wireless communication. As an example of the purposes, a home-use microwave oven uses a microwave in the 2.4 GHz band.
As interference sources affecting wireless communication, various manufacturing apparatuses are installed in a plant. For example, there is a manufacturing apparatus driven by a motor controlled by an inverter. For example, an inverter includes a converter unit converting a commercial power source (AC) into DC and an inverter unit applying variable-frequency AC voltage to a load (such as a motor) by changing an on-off frequency of a switch. Consequently, a rotational speed of a motor can be variably controlled. Noise is generated upon on-off switching of a switch in the inverter. For example, in an inverter using pulse width modulation (abbreviated to “PWM”), a PWM cycle is kept constant, and a motor is controlled by changing an on-off switching time of a switch, that is, a duty ratio. For example, the noise may reach a microwave frequency such as a 2.4 GHz band. Consequently, the noise may degrade quality of wireless communication in a manufacturing facility such as a plant, and an effect may appear as, for example, missing data (such as data sensed by a sensor).
Other interference sources include another wireless system. Wireless systems in compliant with a plurality of wireless standards as described above may be installed in a mixed manner in a plant. In general, wireless systems in compliant with a plurality of wireless standards cannot cooperatively perform wireless control. Accordingly, in each wireless system, another wireless system is mutually handled as noise. Further, even between wireless systems in compliant with the same wireless standard, another wireless system is similarly handled as noise when the systems are not cooperatively controlled. Regardless of a type of interference source, and without being limited to the types described above, an electromagnetic wave from an interference source other than a communication system of interest that may affect wireless communication in the communication system of interest is also herein referred to as “noise.” Note that, since an electromagnetic wave is reflected in a closed space such as a plant, a spot susceptible to noise and a spot not susceptible to noise exist depending on a location.
A wireless LAN operates in such a way as to start communication after sensing whether an interference wave such as noise is generated or whether another communication terminal is performing communication, by use of, for example, carrier sense multiple access/collision avoidance (CSMA/CA).
Further, many technologies for avoiding noise degrading communication quality or interference from another communication traffic are proposed, regardless of a wired network or a wireless network.
PTL 1 discloses a network monitoring unit that estimates network status and a configuration that maintains voice communication by holding down an amount of data transmitted to a network by performing, based on the network status estimated by the network monitoring unit, compression of voice data, increase of a payload length of a packet for reducing a total amount of transferred data, change of a compression codec of voice data, and the like.
Further, as a configuration for avoiding an effect of electromagnetic noise and preventing degradation of communication quality, PTL 2 discloses an apparatus including a temporary storage unit that temporarily stores a packet received from a network, a monitoring unit that detects an anomaly of an Internet Protocol (IP) packet being caused by an effect of electromagnetic noise, and an estimation unit that estimates a generation pattern of electromagnetic noise. The apparatus operates in such a way as to store an IP packet received from one network (referred to as a network A) into the temporary storage unit when transmitting the IP packet to another network (referred to as a network B), estimate a timing at which electromagnetic noise is not generated, based on a generation pattern of electromagnetic noise, the generation pattern being estimated by the estimation unit, and transmit the IP packet at the timing at which electromagnetic noise is not generated.
PTL 3 discloses a system that, by use of a flag indicating that other traffic is being generated, reduces a packet size when the flag is ON.
PTL 4 discloses a wireless communication apparatus that suppresses interference with another communication and improves data transmission efficiency. In the wireless communication apparatus, a signal sensing unit senses power of a space radio wave signal at the same frequency channel as a plurality of testing packets and outputs sample data of the space radio wave signal. A calculation processing unit converts sample data into time-series sample data being data acquired by plotting the sample data on a time-series basis. When determining that a packet collision caused by interference between a plurality of testing packets and another communication exists, based on time-series sample data, a collision detection unit calculates a packet collision rate from a packet collision count and a transmission count of the plurality of testing packets. A control unit adjusts a parameter for data transmission performed by a data transmission-reception unit, based on a calculation result of the collision detection unit, and when detecting that interference exists in wireless communication, the control unit changes a channel, a packet size, and a communication route.
PTL 5 discloses a communication method of generating a packet by dividing data by a packet size related to a packet loss count and transmitting the packet to a short-distance wireless communication apparatus.
PTL 6 discloses a communication apparatus that detects noise on a transmission line by a noise detection means and performs, by a transmission control means, control in such a way that a data transmission speed of each packet is increased depending on an amount of noise on the transmission line, the noise being detected by the noise detection means. The communication apparatus includes an error detection means that detects an error rate on a transmission line and the transmission control means that performs control in such a way that an amount of data in each packet is decreased depending on an error rate on the transmission line, the error rate being detected by the error detection means.
PTL 7 discloses a transmitting terminal including a parameter calculation unit that, by use of: a characteristic that needs to be guaranteed with respect to a delay time required for a packet to reach from the transmitting terminal to a receiving terminal; and a network state (a transmission delay, a packet loss rate, a transmission bandwidth, and a packet size), determines a predetermined number of packets, the predetermined number of packets being encoded and being transmitted to the receiving terminal.
PTL 8 discloses a method including the following steps. When a packet loss above a first threshold value does not exist, a congestion avoidance algorithm increases an amount of redundancy information inserted to a transmission data stream and temporarily increases a data rate of the transmission data stream. The algorithm receives channel information including a packet loss from the network and monitors the packet loss in order to determine whether or not the network can support a higher data rate of the transmission data stream. When the packet loss stays below a second threshold value, the algorithm increases a bit rate of a media stream while maintaining a higher data rate of the transmission data stream.
NPL 1 discloses an access point including a redundancy function providing redundant communication paths with a 2.4 GHz band and a 5 GHz band by two wireless LAN modules. One wireless LAN module (wireless LAN module A) is used as a wireless access point function, and the other wireless LAN module (wireless LAN module B) is used for investigating a wireless environment. The wireless LAN module B monitors quality of every communication channel in the 2.4 GHz band and the 5 GHz band, and when communication quality of a channel used by the wireless LAN module A is degraded, the wireless LAN module B functions as a wireless access point function using a channel with higher communication quality. Subsequently, the wireless LAN module A functions for investigating the wireless environment. Consequently, high-quality wireless communication is provided.